Merge branch 'jc/update-instead-into-void'

A push into an unborn branch, with "receive.denyCurrentBranch" set
to "updateInstead", did not check out the working tree as expected.

* jc/update-instead-into-void:
  push-to-deploy: allow pushing into an unborn branch and updating it

builtin/receive-pack.c

@@ -743,6 +743,22 @@ static int update_shallow_ref(struct command *cmd, struct shallow_info *si)
 	return 0;
 }
 
+/*
+ * NEEDSWORK: we should consolidate various implementions of "are we
+ * on an unborn branch?" test into one, and make the unified one more
+ * robust. !get_sha1() based check used here and elsewhere would not
+ * allow us to tell an unborn branch from corrupt ref, for example.
+ * For the purpose of fixing "deploy-to-update does not work when
+ * pushing into an empty repository" issue, this should suffice for
+ * now.
+ */
+static int head_has_history(void)
+{
+	unsigned char sha1[20];
+
+	return !get_sha1("HEAD", sha1);
+}
+
 static const char *push_to_deploy(unsigned char *sha1,
 				  struct argv_array *env,
 				  const char *work_tree)
@@ -755,7 +771,7 @@ static const char *push_to_deploy(unsigned char *sha1,
 	};
 	const char *diff_index[] = {
 		"diff-index", "--quiet", "--cached", "--ignore-submodules",
-		"HEAD", "--", NULL
+		NULL, "--", NULL
 	};
 	const char *read_tree[] = {
 		"read-tree", "-u", "-m", NULL, NULL
@@ -782,6 +798,9 @@ static const char *push_to_deploy(unsigned char *sha1,
 	if (run_command(&child))
 		return "Working directory has unstaged changes";
 
+	/* diff-index with either HEAD or an empty tree */
+	diff_index[4] = head_has_history() ? "HEAD" : EMPTY_TREE_SHA1_HEX;
+
 	child_process_init(&child);
 	child.argv = diff_index;
 	child.env = env->argv;

t/t5516-fetch-push.sh

@@ -1437,8 +1437,22 @@ test_expect_success 'receive.denyCurrentBranch = updateInstead' '
 		test $(git -C .. rev-parse HEAD^^) = $(git rev-parse HEAD) &&
 		git diff --quiet &&
 		test fifth = "$(cat path3)"
-	)
+	) &&
 
+	# (5) push into void
+	rm -fr void &&
+	git init void &&
+	(
+		cd void &&
+		git config receive.denyCurrentBranch updateInstead
+	) &&
+	git push void master &&
+	(
+		cd void &&
+		test $(git -C .. rev-parse master) = $(git rev-parse HEAD) &&
+		git diff --quiet &&
+		git diff --cached --quiet
+	)
 '
 
 test_expect_success 'updateInstead with push-to-checkout hook' '
@@ -1501,6 +1515,45 @@ test_expect_success 'updateInstead with push-to-checkout hook' '
 		test "$(cat path5)" = irrelevant &&
 		test "$(git diff --name-only --cached HEAD)" = path5 &&
 		test $(git -C .. rev-parse HEAD) = $(git rev-parse HEAD)
+	) &&
+
+	# push into void
+	rm -fr void &&
+	git init void &&
+	(
+		cd void &&
+		git config receive.denyCurrentBranch updateInstead &&
+		write_script .git/hooks/push-to-checkout <<-\EOF
+		if git rev-parse --quiet --verify HEAD
+		then
+			has_head=yes
+			echo >&2 updating from $(git rev-parse HEAD)
+		else
+			has_head=no
+			echo >&2 pushing into void
+		fi
+		echo >&2 updating to "$1"
+
+		git update-index -q --refresh &&
+		case "$has_head" in
+		yes)
+			git read-tree -u -m HEAD "$1" ;;
+		no)
+			git read-tree -u -m "$1" ;;
+		esac || {
+			status=$?
+			echo >&2 read-tree failed
+			exit $status
+		}
+		EOF
+	) &&
+
+	git push void master &&
+	(
+		cd void &&
+		git diff --quiet &&
+		git diff --cached --quiet &&
+		test $(git -C .. rev-parse HEAD) = $(git rev-parse HEAD)
 	)
 '